III.—A Wind-Worn Pebble in Boulder Clay

Extract

Nearly forty years ago Mr. R. D. Darbishire obtained from the Glacial gravel of Bowdon, Cheshire, a facetted pebble, which I afterwards had the privilege of describing (Proc. Geol. Assoc., xvi, pp. 396–420, pl. xi, June, 1900). The conclusion then reached was that the pebble bore witness to the action of blown sand at a period during or not long before the deposition of that particular bed of Glacial Drift. Since, however, the pebble was in other respects similar to pebbles common in the Bunter beds, and since some had supposed the existence of desert conditions in those Triassic times, there remained the possibility, and to some minds the probability that the pebble in question had received its facets during the Triassic epoch. It may, therefore, be useful to draw attention to a distinctly facetted pebble from the Boulder Clay within twenty -five miles of Bowdon. This was found by Mr. W. D. Brown, of Burscough (north-west of Ormskirk, Lancashire), at about three feet below the surface in the Burscough Brick Company's clay pit. The pebble is composed of a greyish micaceous sandstone, weathering yellow, and too coarse and loose in texture to be capable of taking a polish. The facets on it, however, are clearly cut, and do not appear to have undergone much, if any, subsequent abrasion. Its greatest length is 37.5 mm.; greatest width, 29.5 mm.; greatest thickness, 7.9 mm. What was probably its uppermost surface, during the latest period of its exposure to wind action, shows a flat central area, of irregularly pentagonal outline, surrounded by five facets, of which one is slightly concave, one rather irregular, two slightly convex, and the last markedly convex and probably retaining the original curved surface of the pebble.